Recently, a European transport project has been carried out among several fusion devices for studying the possible link between the mean radial electric field (E r ), long-range correlation (LRC) and edge bifurcations in fusion plasmas. The main results reported in this paper include: (i) the discovery of low-frequency LRCs in potential fluctuations which are amplified during the development of edge mean E r using electrode biasing and during the spontaneous development of edge sheared flows in stellarators and tokamaks. Evidence of nonlocal energy transfer and the geodesic acoustic mode modulation on local turbulent transport have also been observed. The observed LRCs are consistent with the theory of zonal flows described by a ‘predator–prey’ model. The results point to a significant link between the LRC and transport bifurcation. (ii) Comparative studies in tokamaks, stellarators and reversed field pinches have revealed significant differences in the level of the LRC. Whereas the LRCs are clearly observed in tokamaks and stellarators, no clear signature of LRCs was seen in the RFX-mod reversed field pinch experiments. These results suggest the possible influence of magnetic perturbations on the LRC, in agreement with recent observations in the resonant magnetic perturbation experiments at the TEXTOR tokamak. (iii) The degree of the LRCs is strongly reduced on approaching the plasma density-limit in tokamaks and stellarators, suggesting the possible role of collisionality or/and the impact of mean E r × B flow shear on zonal flows.

The mechanism governing the impact of the mass isotope on plasma confinement is still one of the main scientific conundrums facing the magnetic fusion community after more than thirty years of intense research. We have investigated the properties of local turbulence and long-range correlations in hydrogen and deuterium plasmas in the TEXTOR tokamak. Experimental findings have shown a systematic increasing in the amplitude of long-range correlations during the transition from hydrogen to deuterium dominated plasmas. These results provide the first direct experimental evidence of the importance of multiscale physics for unraveling the physics of the isotope effect in fusion plasmas. Introduction.-There is clear experimental evidence that at comparable plasma discharge parameters deuterium (D) discharges have improved confinement properties as compared with hydrogen (H) ones [1,2]. The isotope effect has been observed in many different tokamaks under different plasma conditions with a degree of confinement improvement in energy, particle, and momentum depending on plasma regimes. Interestingly, the isotope effect seems to be weaker, and eventually reversed, in stellarators than in tokamaks [1,2]. Understanding the physics of the isotope effect in plasma transport and confinement remains a fundamental open question confronting the fusion community since more than 30 years of intense research with direct impact in the confinement properties of fusion D-T reactors.Considering that the characteristic step size of collisional transport and turbulent structures both increase with the plasma gyroradius s [3], increasing the mass of the isotope would imply a deleterious effect on transport. Then assuming an estimation of the plasma diffusivity (D 0 ) as the ratio of the square of a characteristic radial scale length (L r ) over a characteristic time scale ( c ), D 0 / L 2 r = c , the isotope effect is a counterintuitive phenomenon if the typical radial length scales as L r % s . In addition, while Bohm and gyro-Bohm behaviors are widely used to describe the empirical confinement time, they have, however, the wrong isotopic mass dependence.Contemporary studies of transport phenomena in a wide range of research areas, including atmospheric flows [4], astrophysics [5], and fusion plasmas [6], have identified a common and fundamental feature of the physics of farfrom equilibrium systems-the ''multiscale'' physics, i.e., how large-scale structures can be developed by small-scale

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The electrostatic potential and density fluctuations have been measured at the edge of TEXTOR tokamak by two toroidally distant Langmuir probe arrays. The geodesic acoustic mode (GAM) zonal flows are observed in potential fluctuations with a toroidal and poloidal symmetric structure.The GAM frequency, , changes monotonically with the local temperature and is close to the frequency-dispersion predicted by theories. Bispectral analysis shows clear nonlinear coupling between the GAM and broadband ambient turbulence. The GAM packet has a narrow radial extent with ≃ (0.5 − 0.7) cm −1 and exhibits explicitly a radially outward propagation. Furthermore, the radial correlation structure of GAMs and their radial propagation have been investigated in a wide range of parameters by varying plasma density (¯0=(1.5-3.0)×10 19 m −3 ) and edge safety factor (5.0 ≤ ( ) ≤ 5.9). It is found that the magnitude of the GAM correlations reduces remarkably with the increase of the plasma density as approaching to the density-limit, while the radial wavelength of GAMs only decreases slightly in higher density and larger ( ) discharges.With increasing plasma density, the radial propagating phase speed of GAMs is strongly reduced along with the drop of the local temperature. The results provide new evidence on the propagation properties of GAM zonal flows.2